Reducing the size of a dynamically expanding VHD file

All Microsoft virtualization software allows you to manipulate virtual hard disk files (VHDs). The standard operations you can perform are listen in the table below.

* To fixed of same size as source
** To dynamically expanding of same size as source

What is missing here is the ability to shrink disks, either dynamically expanding or fixed. This is something that the built in tools cannot do. Enter the guys at vmToolkit and their VHDResizer. VHDResizer will let you shrink a fixed or dynamically expanding disk, provided you do some preparation.

Notice how VHDResizer has determined that the source VHD is a dynamically expanding disk, and that the destination VHD can be set to either fixed or dynamically expanding. This particular source VHD has a maximum size of 16 GB. Because of this the minimum size we can select for the destination VHD is 16 GB, regardless of disk type. The reason we cannot get the disk smaller than 16 GB is that the partitions or volumes in the VHD take up the entire 16 GB of space, even though the actual physical file is much smaller. This is the very basis of a dynamically expanding disk; the guest operating system sees all the space and can address it, but only blocks that have data on them are written to the VHD file. So we need to shrink the volumes inside the VHD to free up space. I find that the easiest way to do this is with Diskpart.exe and the SHRINK command. As long as there is free space at the end of the volume, SHRINK can reduce the size of the volume. You can see how much you can shrink the volume by running SHRINK QUERYMAX. Then you can use SHRINK to shrink the volume by the maximum space available.

But sometimes there is data at the end of the volume or very close to it, making the size you can shrink a volume by very small. To get around this you need to move the files to the beginning of the volume. The best tool I have found to do this is JKDefrag. By using JKDefrag’s action option 5 (Force together) we can force all the files on the volume together at the beginning of the volume. This will cause fragmentation, but we can deal with that by doing a normal defragmentation run when the volume has reached its desired size.

JkDefrag.exe -a 5 c:

But sometimes even this isn’t enough. However many times you run JKDefrag you will still see data at the end of your volume. Chances are that this is the NTFS Master File Table (MFT) stored in the file $Mft. The MFT also reserves a portion of the volume it calls the MFT Reserved space. This is to guarantee that the MFT has space to grow in, even when the disk is nearly full. The inability to write to or update the MFT would lead to disk corruption and orphaned files, which is the reason for this precaution. But to achieve our goal, we need to move the MFT and the MFT Reserved space blocks from the end of the volume and towards the start of the volume. (The MFT isn’t really at the end of the volume, at least it wasn’t in the beginning. Windows places the MFT around the middle of the volume by default, but since we have shrunk our volume it is now at the end.) The author of JKDefrag says that version 4 of JKDefrag will be able to move and defragment the MFT, but until that version is available we have to use another tool. The best I have found is Raxco PerfectDisk 10. PerfectDisk is commercial software, but you can download a trial version to perform your MFT defragmentation. Remember to get the correct version of PerfectDisk, the Pro version will not install on Windows Server for example. Install PerfectDisk 10 inside your virtual machine and analyze the volume you want to shrink. Select the Boot check-box next to the volume drive letter and reboot the virtual machine.

During the boot PerfectDisk will defragment your volume and move the MFT to around the middle of it. You have no control of where PerfectDisk moves the MFT, it uses its own internal logic to determine that. You probably will also see that the MFT will grow, this is also due to PerfectDisk’s internal optimization settings. After the virtual machine has booted you can again use Diskpart.exe and the SHRINK command to further reduce the size of the volume. You can repeat this process until you have reached your desired size for the volume. Then, finally, you can run VHDResizer and shrink your VHD file, and also convert it at the same time.